1. Field of the Invention
The present invention relates generally to the field of CMOS imagers, and more particularly to an apparatus and method for achieving uniform low dark current with CMOS photodiodes.
2. Description of the Related Art
Visible imaging systems implemented in CMOS have the potential for significant reductions in cost and power requirements, as compared to traditional CCDs. Due to the advantages offered by CMOS imagers, there has been considerable effort to develop active-pixel sensor (APS) devices. Active-pixel sensors can provide low read noise comparable or superior to scientific grade CCD systems. Prior art active-pixel CMOS imagers, however, suffer from unacceptable levels of xe2x80x9cdark current.xe2x80x9d Dark current is electronic signal noise present in the pixel outputs that is independent of the image being sensed and that is present even when the pixel array is not being exposed to an image. In other words, dark current is present even when substantially no light is incident upon the image sensing area.
In most CMOS processes, it is generally difficult to reduce the dark current through process mitigation. Even if certain adjustments could be made to the CMOS processes to reduce the dark current for 0.8 xcexcm or 0.5 xcexcm design rules, as the designs are migrated to 0.25 xcexcm or 0.18 xcexcm CMOS processes, the dark current problems will only increase.
One approach to reduce the dark current is to use a pinned photodiode, as taught by Lee et al. in U.S. Pat. No. 5,625,210, entitled xe2x80x9cACTIVE PIXEL SENSOR INTEGRATED WITH A PINNED PHOTODIODExe2x80x9d issued Apr. 29, 1997 and assigned to the Eastman Kodak Company. The pinned diode approach is undesirable for CMOS applications, however, because the p implant creates a punchthrough barrier. A high bias voltage significantly larger than the standard supply voltage is needed in order to punch through the p device in order to not compromise the dynamic range. Furthermore, without proper process controls, experiments have shown that pinned photodiodes can actually increase the dark current.
Thus, it would be desirable to be able to reduce the dark current associated with CMOS imagers, without relying on process adjustments or pinned photodiodes.
The present invention is an apparatus and method for achieving uniform low dark currents with CMOS photodiodes. A threshold voltage of a reset FET in an active pixel sensor is set to an appropriate value such that the dark current from a photodiode is actively removed through the reset FET. This reduces the dark current by over 3 orders of magnitude as compared to conventional active pixel sensors, without requiring pinned photodiodes.
According to the present invention, the threshold voltage should be set per the simplified expression:       V          T      Optimum        =                    V        x            [              ln        (                              μ            ⁢                          xe2x80x83                        ⁢                                          W                rst                                            L                rst                                      ⁢                          C              ox                        ⁢                          V              x              2                                            2            ⁢                          J              dark                        ⁢                          A              det                                      )            ]        -          V      x      
where Vx=nkT/e
n is the ideality of the reset MOSFET (about 1.5 for well-designed mixed-signal CMOS processes), Jdark is the dark current density for the photodetector, Adet is the photodetector area, xcexc is the MOSFET mobility, Wrst is the width of the reset MOSFET, Lrst is the length of the reset MOSFET, and Cox is the MOSFET capacitance density. For example for 0.5xcexcm CMOS where we assume a subthreshold ideality of 1.5 along with Jdark=2 nA/cm2, Adet=8.78xc3x9710xe2x88x927 cm2 (28% optical fill factor for 5.6xcexcmxc3x975.6xcexcm pixel), electron mobility xcexc600 cm2/V-sec, Wrst=1.3xcexcm, Lrst=m, 1xcexcm, and Cox=4.6xc3x9710xe2x88x927 F/cm2 for 90xc3x85oxide thickness, the optimum threshold voltage should be about 0.760V to null the dark current at 295 K.
Thus, by raising the threshold voltage Vt for the reset transistor from 0.55 to approximately 0.76 volts for a 0.5 xcexcm process, the typical CMOS photodiode dark current of about 1 nA/cm2 can be balanced off. This results in a zero net dark current, independent of the integration time. The present invention is similarly applicable to other CMOS processes (0.25 xcexcm, 0.18 xcexcm, etc.), and a respective optimized threshold value for these other processes can be determined by using the standard MOSFET electrical parameters associated with each process.